1. Field of the Invention
The present invention relates to the preparation of phenol and, more especially, to the preparation of phenol by direct hydroxylation of benzene.
2. Description of the Prior Art
Various processes for the synthesis of phenol, in particular from benzene, have long been known to this art (Kirk-Othmer/Thurman C., vol. 17, pp. 373-384).
For example, benzene is contacted with propylene in the presence of a Lewis acid, such as aluminium trichloride, or in a heterogeneous medium in the presence of a phosphated silica.
The cumene produced is oxidized in a second step to cumene hydroperoxide which in turn is converted to phenol in a sulfuric medium. This technique is still employed today, but requires three stages and involves peroxide intermediates which the industry would prefer to avoid.
Those skilled in this art have long sought, essentially unsuccessfully, to introduce the desired hydroxyl group directly onto the benzene nucleus. The only publication describing the direct introduction of a hydroxyl group onto a benzene nucleus is believed to be the paper by Iwamoto, published in the Journal of Physical Chemistry, 87, 6 (1983).
Such benzene hydroxylation reaction is carried out by means of nitrous oxide (N.sub.2 O) in the presence of a catalyst based on an oxide of a metal of Groups V or VI of the Periodic Table.
Vanadium oxide is the preferred oxide among the oxides of the metals of Groups V and VI of the Periodic Table. It is more preferred to employ such oxide deposited onto a support based on silica, in an amount by weight ranging from 1% to 10% relative to the support. The support preferably is composed of silica, since alumina in most cases causes the formation of a mixture of carbon oxides.
This particular process was of interest, but the requirement for special catalysts made it relatively unattractive to industry.
Thus, serious need continues to exist in this art for a process for the direct hydroxylation of the benzene nucleus on a simple and readily available substrate.